Cerumen is presently the main cause of damages to, or of a malfunctioning of, an in-the-ear hearing aid. Indeed the duct communicating with the receiver is directed with a substantially straight course towards the internal end of the ogival body and substantially parallel to the acoustic meatus. Thus cerumen may easily get into the duct. While it is possible, on the one hand, to obviate any occlusion which may occur in the duct by periodically disassembling the hearing aid and removing any cerumen therefrom, it is impossible, on the other hand, to protect the receiver against the chemical action of cerumen which produces irreversible damages. Particularly for the users of in-the-ear hearing aids, also the action due to liquid secretions having various causes, such as a simple cold, an otitis, an allergy and increased sweating promoted by an occlusion of the acoustic meatus owing to the presence of the hearing aid, adds to the action of cerumen. Thus, the mean life of the receiver, which under normal conditions ranges between six months and one year, is presently reduced to a few days or a few weeks, despite the various attempts to obviate the disadvantages, such as, for example, by the provision of small grids at the outlet of or within the duct communicating with the receiver or of a small hole provided between the receiver and the outlet of the apparatus.